The info is now fetched by the server on status change. This way the
info can be used in the notifications and also improves performance when
listing a huge amount of applications in the ui.

closes #349

With this commit you can now associate your applications with custom
metadata using `spring.boot.admin.client.metadata.*` sensitive values
are recognized by key and are masked in the http interface.

This can be useful in the future if there are instance specific settings
which should be used by the admin server, e.g. custom notification
recipients.

For now there is no extra imetadata view in the ui. The values are shown
in the environment view.

From this commit on the source from wich the application was registered
is remembered. We use this information for two things:
 * For applications which wasn't registered via the http-api the remove
   button is hidden.
 * The `ApplicationDiscoveryListener` only removes stale applications
   (those which are not anymore found via discovery) if the were registered
   via the `ApplicationDiscoeryListener`.

closes #328

The StatusInfo has a new details field. The StatusUpdater puts either the
reponse from the health endpoint or the exception message in the details.
This allows us to see the details over time (via the journal) and also to
use `em in a bit more mail notifications.

closes #329

With this commits the server no longer depends on the client. The
classes for the clients are moved to de.codecentric.boot.admin.client
package.
Since the perspective on applications is totally different from client
and server the model has been split apart.

What means this for users:
1) The client is no longer a transitive dependency for your admin server
you may need to add it explicitly to your dependencies, respectively
jolokia-core in case you're using discovery and want to use the jmx-bean
interface

2)In case you have cutomised parts of the client you need to reflect the
package change.

fixes #332